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Adsorption of Aluminum by a Smectite: I. Surface Hydrolysis During Ca²⁺-Al³⁺ Exchange¹

ABSTRACT

The exchange of Ca by Al in montmorillonite suspensions adjusted to low pH (4) was investigated by measurement of Ca, Al, and pH in the solution phase before and after the addition of clay. Adsorption of Al was accompanied by a release of protons to solution, indicating that the clay surfaces promoted hydrolysis of Al³⁺ when only partial coverage of exchange sites by Al had occurred. Calculations showed a two- to nine fold increase in the degree of hydrolysis at the surface relative to the value for solution. In suspensions with pK_sp values near 33.3, Al(OH)₃ was precipitated on interlamellar surfaces. However, at lower pH, the predominant form of adsorbed Al was Al³⁺, with hydroxy -Al monomers and/or polymers also present. The preference of the ions for the clay surface followed the order: hydroxy-Al>Al³⁺>Ca²⁺>H₃O⁺. The exchange systems were complicated by proton uptake by the montmorillonite, resulting from H₃O⁺-Ca²⁺ exchange as well as proton adsorption by edges concurrent with slow mineral dissolution reactions.

¹ Contribution of the Dep. of Agron., Cornell Univ., Ithaca, NY, 14853. Agronomy Paper no. 1203.

² Graduate Research Assistant, Assistant Professor, Undergraduate Chemistry Major, respectively.

Received for publication December 3, 1976. Accepted for publication July 15, 1977.

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